1a.Objectives (from AD-416):
Objective 1: Develop adapted potato clones with enhanced resistance to major potato diseases. Sub-objective 1.A. Characterize the molecular genetic basis for late blight (Phytophthora infestans) resistance in the diploid potato species Solanum bulbocastanum. Sub-objective 1.B. Identify resistance genes/factors present within late blight resistant accessions of the diploid wild potato species Solanum verrucosum. Sub-objective 1.C. Develop adapted potato germplasm with high levels of resistance to the fungal pathogen Verticillium dahliae and determine the genetic basis of resistance. Sub-objective 1.D. Identify sources of resistance to early blight (Alternaria solani), common scab (Streptomyces scabies), and soft rot (Erwinia spp., aka Pectobacterium spp.), and introgress them into S. tuberosum. Objective 2: Evaluate exotic potato germplasm for flavor and nutritional components, and introgress valuable genes into the cultivated potato. Sub-objective 2.A. Identify major components of flavor in potatoes and determine the range of variation for those traits in exotic potato germplasm. Relate biochemical variability to sensory analysis data. Sub-objective 2.B. Assess the genetic variability in wild Solanum species for nutritional quality traits including starch composition, antioxidant capacity, and vitamin and mineral levels. Where valuable variation exists, determine the genetic basis of the trait and begin studies to introgress useful genes into the cultivated potato. Objective 3: Examine exotic potato germplasm for resistance to low temperature sweetening and introgress valuable genes into the cultivated potato. Objective 4: Characterize molecular, physiological and environmental parameters that are determinants of potato quality, especially seed vigor and tuber processing quality.
Sub-objective 4.A. Characterize molecular and physiological changes that occur in potato tubers that cause, or are tightly linked to, the accumulation of reducing sugars. Sub-objective 4.B. Determine the genetic and physiological basis of tuber vigor across storage times and environments. Sub-objective 4.C. Characterize water relations and respiration of stored potato tubers and determine the range of variation for these parameters in wild and cultivated potatoes.

1b.Approach (from AD-416):
This project focuses on utilizing wild potato germplasm as a source of genes for traits important to potato improvement, such as disease resistance, flavor, nutritional quality, and low temperature sweetening. We will identify novel resistance to major potato diseases in wild relatives of the potato and introgress that germplasm into the cultivated potato. We will also use molecular genetics to characterize resistance mechanisms in wild species and hybrids with the cultivated potato (Objective 1). In addition, we will screen wild and cultivated potato relatives for flavor and nutritional quality traits, and introgress genes for these traits into the cultivated potato. In parallel, we will determine the biochemical components that can be manipulated to improve flavor and nutrition (Objective 2). We will carry out genetic studies to determine the genetic basis of cold sweetening in wild and cultivated potatoes at the diploid level, and introgress selected germplasm into the cultivated potato (Objective 3). In addition, we will carry out studies to determine the physiological basis of tuber vigor and tuber processing quality (Objective 4). Together, the completion of these objectives will lead to the development of potato cultivars that contain increased genetic diversity, require less management and are highly marketable, leading to increased revenue for the U.S. potato industry.

3.Progress Report:
Obj 1. We have introgressed resistance to late blight, early blight, soft rot, Verticillium wilt, potato virus Y (PVY), and common scab from Solanum species into cultivated potato. Sequence data for the potato ortholog of the tomato Ve gene are being used to develop a molecular marker for Verticillium wilt resistance. We have localized a major gene for PVY resistance to chromosome IX. Diploid and tetraploid populations segregating for both Ve and the PVY resistance genes have been generated and entered in field trials. A germplasm release has been approved for a tetraploid russet clone with common scab and soft rot resistance derived from a wild potato relative. We have also found that variation within RB gene orthologs alters the molecular interaction with the P. infestans effector IPI-O. This interaction allows IPI-O to suppress RB-mediated resistance and alterations in this interaction should impact the durability and the resistance spectrum of RB genes.

Obj 2. We have continued sensory analyses in conjunction with the United States Potato Board (USPB) and have completed additional taste panel evaluations of potatoes grown on organic farms.

Obj 3. We completed evaluations of chip color of cultivated x wild hybrids in greenhouse and field studies. We are finishing assays of invertase, sugar levels, and invertase gene transcript levels in those populations to establish relationships with chip color. Two populations derived from elite cultivars have been genotyped and are being phenotyped to identify genetic loci linked to processing quality traits and to develop molecular markers that can be used to increase the efficiency of potato breeding programs. Data for sugar composition, activity of enzymes of carbohydrate metabolism, and gene expression in cold-stored tubers of S. pinnatisectum have been analyzed. Carbohydrate metabolism with respect to reducing sugar accumulation is different in S. pinnatisctum than in cultivated potato. An examination of potato tubers modified to have very low expression of the vacuolar acid invertase gene has shown that expression of other key genes of carbohydrate metabolism are unaffected in low invertase lines.

Obj 4. An experiment describing the detrimental effects of transient water stress on sugar composition and defect formation in russet potatoes is being repeated. An experiment describing the detrimental effects of transient heat stress on sugar composition and defect formation in chip potatoes is being repeated. An evaluation of seed tuber gibberellin treatments on seed tuber vigor as determined by rates of emergence, canopy closure timing, and tuber yield is being repeated. Responses of potato tubers to ethylene in the storage environment are being evaluated in terms of varietal differences in respiration rates.

4.Accomplishments
1.
Host pathogen interactions in late blight disease. Despite intensive breeding efforts, potato late blight, caused by the oomycete pathogen Phytophthora infestans, remains a threat to potato production worldwide because newly evolved pathogen strains have consistently overcome major resistance genes. The potato RB gene, derived from the wild species Solanum bulbocastanum, offers the unique opportunity to directly introduce a late blight resistance gene into all current potato cultivars and future advanced germplasm. This variation is now being used to predict whether new strains of the pathogen can overcome host plant resistance.

2.
Reducing sugar accumulation in stem-end chip defect formation. Potatoes used for the production of chips must maintain very low concentrations of tuber reducing sugars and have few defects in order to be acceptable to chip processors. Potatoes that do not meet specifications may be rejected at processing plants. This increases the financial risk to growers and may limit supply to processors. Stem-end chip defect, which is characterized by dark fried color in chips near the vascular tissues at the tuber stem end, has cost United States chip potato growers millions of dollars over the past decade. Our research indicates that infection with the pathogen that causes Verticillium wilt also contributes to stem-end chip defect formation. These data are being used to guide field management decisions for chip potato production.